Metallic bone plates and screws have been used for sometime in osteosynthesis to approximate fractured or broken bones in the body. These plates are generally made of materials such as stainless steel, chrome cobalt, titanium and various alloys of such metals. The bone plates are used to hold fractured bones in position so that they may heal in a proper manner. The bone plates offer advantages over the immobilization of the bone using only simple casting techniques. The use of internal fixation eliminates long periods of casting and allows early active joint movement which provides greater or earlier mobility to the patient.
It has been suggested that it would be desirable to form such bone plates from materials which would be absorbed by the body to eliminate the necessity of a second surgical procedure to remove the bone plates after the bone has healed. An absorbable polymer capable of being fabricated into a bone fixation device or bone plate is disclosed in U.S. Pat. Nos. 4,539,981 and 4,550,449.
Bone plates fabricated from metal have been made in various designs. Generally, the design consists of a bar of the particular metal which is curved on the surface which will be placed against the bone. The plate has a number of screw holes in the plate and screws are introduced through the holes to secure the plate to the bone U.S. Pat. No. 3,463,148 discloses a metallic bone plate which has a substantially constant cross-sectional area. The plate has screw holes through the plate which are spaced on either side of a longitudinal center line. The metal in the area of the screw holes is thicker than the area of the metal between the screw holes.
U.S Pat. No. 4,219,015 discloses a metallic bone plate in which the bending resistance moment, W=I/e, is relatively constant throughout the plate and the stress is variable. Specifically, the lower limiting value of the bending resistance moment is, at most, 30% smaller than the upper limiting value. In the plate of the present invention the stress is relatively constant and the bending resistance moment is variable.
U.S. Pat. No. 4,429,690 discloses a metal plate for the fixation of broken bones comprising two longitudinal bars joined by an array of humped bridges or crossed brackets evenly spaced along the length of the bars and having holes to set cortical screws. The design of this plate is indicated to resists fracture to a greater degree than the plates previously used.
All of the above mentioned bone plates are designed to be fabricated from a relatively strong metal, such as stainless steel, chrome cobalt or titanium. The absorbable polymer from which the present bone plates are made does not have the strength of these metals. The strength of the absorbable polymer is significantly less than the strength of the metal from which the metallic bone plates are fabricated and, for that reason, the design of the metallic bone plates are not necessarily usable in a bone plate fabricated from an absorbable polymer. Although it would be theoretically possible to simply increase the thickness of an absorbable bone plate to compensate for the difference in strength when compared to a metallic bone plate, such simple modifications are not desirable. It is necessary to minimize the thickness of any bone plate so that the plate will not sit too high on the bone and cause difficulty in the coverage of the bone plate with soft tissue following a surgical procedure. If the plate is too thick, it simply cannot be used. Similarly, anatomical restrictions also limit the width of a bone plate. The bone plate cannot be too much wider than the width of the bone for which it is designed to repair.